This study systematically investigates the very high cycle fatigue (VHCF) behavior of an extruded Mg-Gd-Y-Zn-Zr alloy at room temperature and 150 ℃, with particular emphasis on elucidating the mechanisms of crack initiation under high temperature conditions. The results show that the alloy exhibits superior fatigue performance at 150 ℃ compared to room temperature. At ambient conditions, cracks predominantly initiate along basal slip systems, whereas elevated temperatures activate prismatic slip and twinning, resulting in more diverse crack initiation mechanisms. Furthermore, elevated temperature facilitates a more uniform distribution of plastic deformation, thereby enhancing fatigue resistance. Notably, a dense ZnO layer forms on the alloy surface at 150 ℃. Compared to the brittle and crack-prone MgO layer, this ZnO layer offers more effective surface protection and significantly delays crack initiation. The enhanced fatigue resistance is thus primarily attributed to the combined effects of the protective ZnO layer and the uniform plastic deformation. These findings provide theoretical guidance for optimizing the high-temperature fatigue performance of rare-earth magnesium alloys.

Magnesium alloys with a long-period stacking ordered (LPSO) structure usually possess excellent static strength, but their fatigue behaviors are poorly understood. This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue (VHCF) regime. The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction. The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys, contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys. Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface, which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior.